1. Field of the Invention
The present invention relates to RF amplifiers and more particularly to multi-octave RF power amplifiers.
2. Descriptions of the Prior Art
Broadband RF power amplifiers often use a single transistor in the final amplifier stage. The broadband tuning and the normal drop in RF performance of the transistors at high frequencies result in reduced power output from the amplifier at the high frequency end of the amplifiers operating band.
To increase the power of the amplifiers beyond that of a single transistor, paralleling of transistors stages by means of wide band coupling networks is often employed. Commonly used methods of paralleling do not eliminate the problem in that the power roll off from low to high frequency remains, only at a higher power level than for a single stage. It is difficult in the usual paralleling methods to add in power only at the high frequency end of the band. There are in addition problems with the circuitry commonly used in the usual paralleling methods.
A network which can be used for paralleling transistor amplifier stages is a four-way, in-phase power divider-combiner. This network has the advantage of broadband operation and isolation between the paralleled transistors. Each of the paralleled transistor stages is isolated from the rest to prevent a single stage from accepting all the drive power. This arrangement avoids destruction of transistors which otherwise might occur.
The bandwidth of a four-way divider-combiner can be 1 to 1000 Mhz or greater which is approximately 10 octaves. This is excellent frequency coverage, but there are some drawbacks to this network.
It can only be used to combine four transistors. In many cases, it is only necessary to combine two transistors. It is currently more difficult to produce a high power, two way combiner than it is to produce a four-way combiner. When the power level of a four-way divider-combiner exceeds two to three watts, the price generally increases by a factor of ten because simple ferrite coils can no longer be used to handle the power load. Usually semirigid lines with higher power handling capability must be used. This adds significantly to the cost of an amplifier incorporating such devices. In addition, the VSWR of the transistors stage is, at least in part, reflected in the output of the amplifier. The appearance of the transistor's output impedance at the amplifier's output port is often a problem. It is difficult to present through a matching circuit, an optimum load to the transistor and, at the same time, translate back through the same matching circuit, the output impedance of the transistor to the amplifier output port and have it appear as a standard impedance value of, say, 50 ohms.
There is a coupling device which can combine the output power of two transistors and at the same time help mask the actual impedance of the transistor output stages by presenting a standard output impedance to the output port of the amplifier. This device is the quadrature coupler. Typically, in amplifier employing quadrature couplers, two amplifier stages are placed between two quadrature couplers as shown in FIG. 1A below and, also in U.S. Pat. No. 3,911,372 to Seidel.
Although the quadrature coupler provides the important advantage of masking transistor VSWR, there are a number of practical disadvantages which must be considered. Unfortunately, most low cost quadrature couplers are octave devices, seriuosly limiting the frequency range of the amplifier. Although there are multi-octave quadrature couplers, typically made in stripline, they are usually costly, and large for applications below 500 Mhz, making their use in production amplifiers relatively rare.
It is therefore important to provide an improved system which overcomes both disadvantages of the prior art in that the improved system should function with a low cost octave coupler over multiple octave bands, while retaining the advantage of combining two transistor stages, and presenting a low VSWR over the entire range of the coupler.